Clarification #1: Nevertheless, Tefilah helps within forty days to turn the fetus into a boy or a girl ...

Source: As we learned in B'rachos 'Forty days before the formation of the baby, one can Daven that one's wife should give birth to a boy'.

Clarification #2: And even one's first match can be switched through the medium of Tefilah ...

Source: As the Gemara says in Mo'ed Katan 'Although one may not get married on Yom-Tov, betrothal is permitted, to pre-empt somebody else from 'getting her' via Tefilah.

Question: How does he then establish the Gezeirah-Shavah of "Pera" "Pera"?

Answer (Part 1): It therefore seems what the Gemara means is 'Let us say that they are not permitted to grow a thirty-day growth, which is the Shi'ur 'Pera', but that they are obligated to cut their hair on the twenty-ninth day.

Answer (Part 2): And the Gemara answers that had the Pasuk written 'Lo Yeshalchu Pera', it would indeed imply that they are not permitted to grow a thirty-day growth; But now that it writes "Pera" before "Lo Yeshalechu", it implies that one may not allow more than Pera to grow.

Question: By establishing the Hekesh of Yechezkel, Rav Ashi contradicts what he himself said earlier, that Kodshim with long hair do not invalidate the Avodah?

Answer (Part 1): Rav Ashi's answer here ('Gemara G'miri ... ') actually answer both Kashyos - 'Until Yechezkel came - who said it'? And the earlier Kashya that since it is a Halachah le'Moshe mi'Sinai, we can say that the Halachah was presented with regard to Misah, but not with regard to disqualifying the Avodah.

Answer (Part 2): And when Yechezkel supported the Halachah with a Pasuk, he did so with regard to the Halachah, with the result that we restrict the support from the Hekesh to the aspect of Misah exclusively.

Answer (Part 3): Whereas above, before we knew of the Halachah, we thought that the Hekesh from Peru'even if to Shesu'yei Yayin is total, based on the principle 'Ein Hekesh le'Mechtzah' ('There is no such thing as half a Hekesh').

Clarification: Like spelt (Kusemes) which grows like that - the tip of one grain reaches the base of the next one.